Density functional theory calculations have been performed to study the combined interaction of oxygen and chlorine with the Cu(100) surface. We found the presence of atomic chlorine increases the stability of molecular oxygen adsorption, and that the barrier required to dissociate the oxygen molecule in the presence of chlorine is three times larger than the dissociation barrier of molecular oxygen on the clean Cu(100) surface. In addition, chlorine monoxide was generated on the surface when molecular oxygen was adsorbed horizontally into a hollow site immediately adjacent to atomic chlorine. Our calculations indicate that while chlorine is easily adsorbed dissociatively on the clean Cu(100) surface, it is stable in the molecular form in the presence of atomic oxygen. The presence of chlorine leads to the production of subsurface atomic oxygen and enables an oxygen atom to go into the Cu bulk with a small activation energy barrier.